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The sea wall at the Jefferson Memorial has sunk almost a foot in places since the monument was built, and the rate seems to have increased in recent years, according to a year-long study commissioned by the National Park Service.

As a result, the sea wall, in the Tidal Basin, should be reinforced with pilings driven through the mud flats and anchored in bedrock far below, a project that would probably cost more than $10 million, a Park Service spokesman said.

Park Service officials said they would study the report and conduct further investigations before deciding on a course of action.

The 32,000-ton memorial does not appear to be sinking. But the report urges continued monitoring of the 18-acre complex to understand what is happening in the ground. One engineer said that if nothing is done, the problems will worsen.

The Park Service says the memorial is safe. The agency "wants to reassure all Washingtonians and all national and international visitors that the memorial remains open," spokesman Bill Line said yesterday. "It is absolutely safe and sound."

The study -- conducted by HNTB Federal Services of Washington, Schnabel Engineering of West Chester, Pa., and three other companies -- began in the fall of 2006 after officials noticed cracks in the surface of the memorial's north plaza and spots where the sea wall was slipping into the Tidal Basin. The north plaza also seems to be moving -- not down, but out toward the Tidal Basin, the study says.

The study, which was completed this year and released yesterday by the Park Service, found that the sinking was mostly caused by the compression over time of the soft soil under the wall. It says that the cause of the more rapid rate of sinkage was unknown but that it might be related to changes in groundwater levels.

"It's a significant issue," said Nathan James, a project manager with HNTB Federal Services. "It's something we all believe, if it isn't addressed, there's nothing to stop it, nothing to arrest the problem."

The domed memorial, one of Washington's most photogenic monuments, was built in the late 1930s and early '40s. It was constructed atop six inner rings and five outer rings of pilings and pipes driven into an artificial mud flat. At least one support was sunk 138 feet to the bedrock, according to the Park Service.

Periodic settlement has been an issue since before the memorial was dedicated. In 1941, some of the supports under the main steps began shifting and were temporarily lashed together with steel cable and turnbuckles, according to a 1965 engineering report.

Settlement also caused supports under the steps to bend slightly, the report says. The steps moved so often that park rangers once kept a tool at the site to realign them. Lesser settlement problems cropped up around the memorial during the 1940s and '50s.

By the 1960s, the plaza, which was not supported by pilings, had begun to sink -- by as much as three feet, according to a Park Service review. A $1.1 million project was undertaken from October 1969 to December 1970 to bolster the area with pilings and other supports.

The sea wall along the memorial's northern rim had always appeared sound. Unlike the plaza, the sea wall was supported in its original construction by timber pilings, some of which were set at an angle to add support.

The study recommends the installation of at least 50 new, thinner piles either through or around the sea wall to the bedrock. Exploratory drilling has begun to test the sea wall, James said.

During any repairs, the Park Service would probably create alternate entrances to the memorial to make sure it remains open, said Line, the agency spokesman. "Our intention would be to never close the Jefferson Memorial to visitors."